Actuator seek acoustic noise is a problem in small form-factor DASD's particularly in the lap-top computer environment as acoustic shielding materials can-not be easily deployed due to severe space, weight, heat dissipation, and cost constraints. The problem is acute in compact DASDs, as the physical dimensions of the actuator components are small, thus pushing the structural vibration modes into the acoustic range sensitive to the human ear. The problem rapidly becomes worse in high-performance files as the access times are pushed down drastically and more high frequency acoustic noise components are present.
Conventional DASD designs generally use a digital servo-control system to provide head motion for operations starting from any arbitrary position on the disk to the desired track where the data is read or written. The head movement includes a track-access phase where the head is accelerated to high velocities and moved over a relatively long distance prior to deceleration. The track-access phase requires large actuator forces which excite the structural resonant modes of the entire head-disk assembly (HDA) leading to high-pitched acoustic noise. During the track-access phase, the actuator servo-system follows a velocity profile, as is more fully explained with reference to FIG. 1.
In FIG. 1, a voltage (or a digital number) representative of the target track position is subtracted from a voltage (or a digital number) representative of present track position information at a node 20 to produce a tracks-to-go signal, The difference voltage is fed to a reference velocity generator 22 to produce a reference voltage for a velocity servo and actuator system 24. The application of the references voltage causes the actuator to move to a new position. A signal representative of position is fed back to block 24 and to junction 20 as noted above.
Referring to FIG. 1A, velocity servo and actuator system 24 includes a summing junction 28 for providing an output proportional to the difference between the reference voltage and the output voltage or a velocity estimator 30. The output of junction 28 is provided as an input to a velocity controller 32. The output of velocity controller 32 is provided as an input to estimator 30. Another input to estimator 30 is the position signal.
The output of velocity controller 32 is also provided as an input to a driver circuit 34 which provides a current output to a head position actuator 36 of sufficient magnitude and proportional to the voltage output of controller 32, so as to control the position of the head 38 as it interacts with the storage medium 40.
With more specific reference to FIG. 1, the target track is denoted by X.sub.Target and the actual head position and velocity are represented by X and V. The reference input to the velocity profile is shown as the tracks-to-go .delta.X which generates a reference velocity V.sub.Ref for the velocity servo. The velocity profile is generally stored as a look-up table in the control processor memory. The velocity profile in general consists of an acceleration phase (A), a constant velocity phase (B) and a deceleration phase (C) as a function of the distance to the target track. Some DASDs do not have a servoed acceleration phase A, where phase B extends to the beginning of seek. (In mid-range products the velocity profile is generated by the microprocessor in real-time by storing the desired set of deceleration parameters.) A typical velocity profile of a low-end DASD with an attempt to reduce "jerk" is also shown in FIG. 1. As can be seen, there are still abrupt discontinuities between phases A, B and C leading to rapid changes in the acceleration/deceleration current at the transition points. This translates into abrupt changes in the driving force for the actuator. The resulting jerks (i.e., time rate of change in acceleration/deceleration) can excite and cause a ringing of the actuator/HDA mechanical resonant modes which may cause acoustic noise. Measurements obtained in 2.5 inch diameter disk drives suitable for portable applications have shown strong correlation between rate-of-change of current and acoustic peaking.
An obvious solution to this problem is to insert a bank of filters, such as, for example, low pass filters between the control processor and the power driver which drives the actuator. This has the effect of reducing the high frequency jerk components in the control command (U, in FIG. 1A) which drives the actuator by means of current from driver 34. However, the filter introduces a significant phase-lag in the control loop. This leads to a loss of phase margin which is a critical parameter of the servo-system and may reduce the stability of the control loop leading to increased actuator settle-out and track misregistration (TMR) problems caused by excessive vibration. Use of notch filters to suppress potential resonance-based acoustic components only partially solves the problem with a cost penalty as well as some phase lag.
Reduction in velocity servo gain is another method that would reduce rate of change of current, but would compromise settle-out. Adding a real or simulated inductance, to a voltage mode driver can potentially improve the acoustic condition. The look-up velocity profile can also be reshaped to "soften" the transitions, but the difficulty with this approach is that excessive storage is required to generate such a profile for the complete range of seek lengths, i.e., from short seeks to long seeks. Further, the software over-head to look ahead for the transition points cannot be predicted efficiently in the time domain for different actuator seek lengths (particularly for those of short seeks). This approach would essentially require the storage of a very large number of individual velocity profiles for different seek lengths.